Wireless communications networks, in which a user equipment (UE) such as a mobile handset communicates via wireless links to a network of base stations or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations of radio access technology. The initial deployment of systems using analogue modulation has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), typically using GERA (GSM Enhanced Data rates for GSM Evolution Radio Access) radio access technology, and these systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), using the UTRA (Universal Terrestrial Radio Access) radio access technology. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposals by the Third Generation Partnership Project (3GPP) of the Long Term Evolution (LTE) system, using E-UTRA (Evolved UTRA) radio access technology, which offers potentially greater capacity and additional features compared with the previous standards.
Note that the term “GERA” is used herein to refer to the radio access technology associated with GERAN (GERA networks), “UTRA” is used to refer to the radio access technology associated with UTRAN (UTRA networks), and similarly the term “E-UTRA” or “LTE” is used to refer to the radio access technology associated with E-UTRAN (E-UTRA networks).
LTE is designed primarily as a high speed packet switched network, and voice services, packet switched voice services and in particular Voice over Internet Protocol Multimedia Subsystem (VoIMS) services are envisaged, whereas previous generation systems such as UMTS support voice services that are primarily circuit switched.
As new technology is introduced, networks are typically deployed which include radio access networks that use a radio access technology according to a recent standard and also legacy radio access networks that use a legacy radio access technology. A user equipment may be typically capable of communication using two or more radio access technologies, so for example the user equipment is able operate using one radio access technology, perhaps offering high capacity, where this is available, but being able to operate using a legacy radio access technology, in those service areas of the network that do not support the other radio access technology, or that do not support preferred features.
In service areas where a radio access network, such as an LTE/E-UTRA Packet Switched (PS) network, does not support voice communication, user equipment may follow a defined procedure to fall back to using another radio access network, such as UTRAN or GERAN, for voice communications, typically falling back to Circuit Switched (CS) voice communications, according to a Voice Call Continuity (VCC) handover procedure.
The Internet Protocol Multimedia Subsystem (IMS) is typically used to control packet switched services offered over the E-UTRA network; control of circuit switched services in a UTRA/GERA network typically involves a mobility controller, such as a Mobility Switching Centre (MSC). The mobility controller typically communicates with the session transfer controller provided by the IMS, during session transfer according to a VCC handover procedure.
A user equipment may be equipped with a single radio transceiver, for reasons of economy or for minimising power consumption, so that simultaneous communication with two radio access networks is not possible. In this case the handover protocol typically uses a break-before-make radio connection during handover. Handover procedures known as Single Radio Voice Call Continuity (SRVCC) procedures have been developed for such situations, and in particular video SRVCC (vSRVCC) procedures for handing over conversational video calls (i.e. calls with voice and video content).
A user making a call may be provided with a customised alerting notification during the alerting phase of a call, which may be referred to as a Customised Alerting Tones (CAT), so that, for example, a calling user may hear an audio clip instead of the standard ring back tone. CAT may also be referred to as Colour Ring Back Tone (CRBT). Multimedia CAT (mCAT) refers to the ability to not only replace the ring back tone by audio, but with other types of media, e.g. video. CAT or mCAT may be provided by the CS (Circuit Switched) network or by the IMS (Internet Protocol Multimedia Subsystem) network.
Typically, the user equipment's capabilities in the circuit switched domain regarding CAT, and in particular mCAT are indicated to a mobility controller of a circuit switched network on initiating a call in the circuit switched network, so that the mobility controller may enable the delivery of the mCAT to the user equipment. However, if a SRVCC procedure, and in particular a vSRVCC procedure takes place for the calling user equipment during the alerting phase of a call, the user equipment may initiate the call in the packet switched domain, so that the indication of the mCAT capability of the user equipment is not sent to the mobility controller. As a result, when the user equipment is handed over to the circuit switched domain, the mobility controller may not know the mCAT capabilities of the user equipment, and so mCAT may not be provided, so that the caller may experience a lack of alerting tone.